Reduction of Flow Separation and Energy Head Losses in Expansions Using a Hump

Abstract

Irrigation channels need expansions to provide a cross-sectional transition. Expansions often cause flow separation, turbulent eddy motions, channel erosion, and flow energy dissipation. The purpose of this paper is to investigate the effectiveness of fitting a hump at the expansion’s bottom in reducing flow separation and energy loss. This paper reports analytical solutions to the problem of flow transition and associated energy head loss, by using the momentum and energy concepts complementarily, as well as experimental results for verifying the solutions. The use of a triangular hump can effectively reduce head loss; the head loss coefficient drops by a factor of up to 4. The mechanism at work is that the presence of a hump forces the flow to accelerate, neutralizes the decelerating effects of expanding width, and creates favorable pressure gradients working against flow separation. Humps with a crest height of 5–9% of the depth of approach flow are recommended, with a good potential to save a substantial amount of flow energy. Researchers can easily incorporate such humps into existing expansions.